In printers and copiers, proper image registration on a page depends upon the correct positioning of the paper within the device. One aspect of paper positioning involves the location of the paper supply within the paper source assembly for the device. The two primary types of paper source assemblies are the removable paper tray and the fixed platform for paper loading. Those paper source assemblies often are used in computer printers and personal copiers. Other paper source assemblies also are known, such as the non-removable drawer-type assembly, which typically is used in larger size copiers and printers. The present invention is suitable for use with these and other types of paper source assemblies, with other means for receiving a supply of printable media, and with media other than paper. However, for convenience, reference is made herein only to the preferred embodiment of the present invention in exemplary applications involving paper and a removable paper tray or fixed platform.
In most types of paper source assemblies or devices, some form of side guide generally is used to help locate the paper in the source. Typically, paper in a removable tray is in the form of a stack of up to 500 individual sheets of paper in either letter (81/2 in. by 11 in.) or legal (81/2 in. by 14 in.) size. A fixed platform generally holds up to 100 of such sheets. However, because other size papers may be used, e.g., A4 paper, or because it may be desirable in some applications to feed various sized media (other than paper) into the printer or copier, e.g., cardstock, transparencies, labels or envelopes, the use of adjustable side guides in paper source assemblies generally is preferred.
One problem associated with the function and operation of paper source assemblies including adjustable side guides is paper pinching. In general, paper pinching results from the positioning of a side guide either adjacent to or too close to a supply of paper. Such a condition might result, for example, from an operator moving a side guide too close to a stack of paper, or from an inability of a paper side guide to accommodate the natural variations in the size of paper from load to load due to, e.g., changing environmental effects and/or the tolerances associated with the manufacture of the paper.
The adverse effects created by paper pinching are significant in terms of the overall operation of the printer or copier. Performance differences can be measured between systems having pinched and unpinched paper sources. Paper skew may be caused by a pinching condition, meaning that ultimately an image may not be properly placed on a page. Pinching also may cause the paper to feed incorrectly, perhaps resulting in a paper jam and/or damage to the machine.
In edge driven systems in particular, in which the paper is fed against a single reference edge that keeps the paper straight all the way through the machine, it is important to avoid the effects of paper pinching. The feed accuracy of the downstream edge driven paper transport system depends upon the drag forces that must be overcome with a particular paper source assembly and edge guide system. Such systems "pull" on only one side of the paper (along the reference edge); thus, if an edge guide provides a frictional drag on the paper, the paper will tend to turn off of its reference edge and travel crooked through the printer, preventing accurate image registration. Moreover, such adverse effects of paper pinching are greatly amplified by the degrees of wrap which the paper goes through in edge driven systems in which the paper travels in a serpentine path.
Most paper source assemblies have at least some limited ability to adjust to various paper sizes. For example, there are three commonly used mechanisms for adjusting the position of side guides in removable paper trays and fixed platforms: fixed detents, a rack and pawl, and a frictional system. In fixed detent systems, the side guides typically include pins that drop into holes in the paper tray or platform to lock the side guide in place. One or more holes are placed at predetermined locations so that the side guides will be positioned to fit various size sheets of paper. Accordingly, the fixed detent system is not universally adjustable to accommodate odd-sized media. The side guides are positioned without regard to possible variations in the actual size of the paper used. Thus, a significant drawback of fixed detents results in part from the paper tolerances associated with the manufacture of the paper, and from paper shrinkage and growth due variable humidity levels. This requires that a nominal clearance of up to 4 mm or more be maintained between the fixed detent and the paper to prevent pinching of the paper, which in turn often results in less than desirable registration accuracy and poor repeatability from load to load due to there being too much or too little gap between the edge guide and the paper.
The second type of side guide adjustment mechanism is a "rack and pawl" system. In such systems the side edge guides typically include a "pawl" or other extension operatively coupled to a rack having a plurality of teeth along its length that define a number of locking positions for the side guide. Thus, this guide system is similar to, but allows greater freedom of adjustment than, fixed detent systems. However, accuracy of side guide adjustment still remains limited by the pitch of the teeth on the rack (typically on the order of 1-2 mm), and although rack-type edge guides allow for better paper constraining, they too are susceptible to paper pinching as described above.
A frictional side guide adjustment system allows the greatest level of freedom in terms of side guide positioning. However, paper pinching remains a problem in these systems too, mainly due to operator error in adjusting the guide too close to the paper supply. Accordingly, with prior paper source assemblies including adjustable side guides there remains a need for an effective means of reducing the problems associated with paper pinching and excessive clearance.